Flared-notch radiators have been traditionally fabricated utilizing printed-circuit board technology. Variations in dielectric constant of the substrate materials used in the fabrication and their low structural strength made this fabrication technology unsuitable for many applications. Therefore, machined-metal flared-notches were developed as a solution to the problems inherent with printed-circuit board flared-notches.
It was discovered experimentally, during radar cross-section (RCS) testing of a flared-notch aperture, that higher order modes of electromagnetic energy are excited between the flared-notch sticks of the array at specific frequencies and propagate along the length of these sticks. Allowing these modes to exist and propagate presents two disadvantages. First, at the mode excitation frequency, large nulls appear in the embedded element pattern of the flared-notch aperture. These nulls will not allow the main beam of an antenna to scan to its designed scan limits thus limiting radar scan performance at higher frequencies. Second, when the energy propagating between the sticks reaches a discontinuity (such as the end of the sticks themselves), some of the energy scatters. This scattering will manifest itself as a resonance phenomena in radar cross section (RCS) measurements of a flared-notch aperture. This resonance phenomena must be eliminated for very-low RCS apertures.
It would therefore be advantageous to provide a flared notch radiator array that suppresses higher order mode excitation without impacting radiation characteristics or array gain.